Field of the Invention
The subject matter disclosed herein relates to rotating machines, in particular but not exclusively to turbomachines, such as gas turbines, steam turbines, axial and centrifugal compressors. More specifically the subject matter relates to improvements concerning the bearings for the rotors of such machinery.
Description of the Related Art
Journal bearings or fluid film bearings are commonly used to support rotating shafts of machines rotors such as shafts of turbine rotors and compressor rotors. FIG. 1 schematically illustrates a turbomachinery rotor 100 comprising a shaft 101 supported in two end bearings 103, 105. The bearings 103, 105 can be journal bearings or so called hydrodynamic or fluid film bearings as schematically shown in FIG. 2. The shaft 101 is supported within a bearing 105, with an oil film 107 filling an annulus or clearance between an outer surface 101A of the shaft 101 and an inner bearing surface 105A of the journal bearing 105. The rotation of the shaft 101 generates an oil movement with a converging oil edge 109 and a diverging oil wedge or diverging oil film 111. The shaft 101 is supported by hydrodynamic pressure generated by the oil motion and having a typical profile 112 like the one shown in FIG. 2. The bearing surface 101A can be formed by a substantially cylindrical surface of the bearing 105 or by a plurality of pivoting bearing pads arranged around the rotation axis of the shaft 101.
Important hydrodynamic coefficients of a journal bearing are stiffness and damping of the bearing. Stiffness is related to the reaction of the lubrication oil film to displacements of the journal or shaft 101 opposed to the movement itself. A downward movement of the shaft 101 is opposed by an upwardly oriented restoring force generated by the oil film. Vertical stiffness is given by the restoring force divided by the displacement of the shaft. Horizontal displacement resulting in a horizontal restoring force generated by the oil film results in an horizontal stiffness of the bearing.
A further important hydrodynamic parameter of journal bearings is damping. This damping coefficient is velocity related. A downward velocity perturbation of the shaft will result in an upward restoring force generated by the oil film, in quite similar manner as a shock absorber. The faster the movement, the larger the opposing force generated by the film. A similar damping effect is provided also in the horizontal direction. Damping is beneficial in terms of suppressing vibrations under certain operating conditions. Larger damping can be obtained with a larger clearance between the bearing surface and the outer surface of the rotating shaft housed in the bearing. However, larger clearances will reduce the stiffness of the bearing, which can be detrimental under normal operating conditions of the rotor. It is sometimes advisable to have maximum damping while crossing critical speeds and maximum stiffness while operating far from critical speeds and close to full speed.